1. Field of the Invention
The present invention relates to improvements of an image projector and more particularly to an image projector capable of displaying a high definition image in spite of a short projection length.
2. Description of the Related Art
Recently, an image projector has been proposed in which three liquid crystal panels are employed correspondingly with Red (R), Green (G) and Blue (B) signals to obtain a high definition image. Therefore, the image projector has such a construction as capable of projecting a composite image consist of the three color images from three liquid crystal panels on a screen through a polarization beam splitter and a tri-color separation and composition optical system having dichroic filters.
It is well known that, in order to obtain a highly magnified image on the screen at a short projection length defined between the projection lens and the screen, it requires a projection lens having a short focal length. In addition, the projection lens has to be positioned in such a manner that the object focal point of the projection lens resides in the proximity of an object to be projected on the screen. These facts are derived from formulas of lens.
FIG. 1 is a explanatory view for explaining formulas of lens in the prior art, wherein the numeral 30 indicates a focusing lens, 31 an object, and 32 an image. The symbol `f` indicates focal length of the lens 30, `F.sub.1 ` object focal point, `F.sub.b ` image focal point, `a` a distance of the object 31 from the center of the lens 30, `b` a distance of the image 32 from the center of the lens 30, `b.f.` a back-focal length defined as a distance between the front surface of the lens 30 and object focal point F.sub.a or a distance between the rear surface of the lens 30 and the image focal point F.sub.b, `h.sub.a ` a height of the object 31, `h.sub.b ` a height of the image 32.
The formulas of lens are expressed as follows: EQU 1/a+1/b=1/f (1) EQU h.sub.b /h.sub.a =b/a=m (2) EQU b=f(1+m) (3) EQU m=1/((a/f)-1).thrfore.(a&gt;f) (4)
The expressions (3) and (4) are derived from the expression (1) and (2), wherein `m` indicates the magnification of the lens 30.
As clearly understood from the expression (3) and (4), it is effective to use a short focal length `f` for the short projection length `b` and is effective to position the object 31 in the proximity of the object focal point `F.sub.a ` for obtaining a large magnification `m`.
On the contrary, the aforementioned image projector requires a projection lens having a long focal length `f` because there are installed the polarization beam splitter 24 and a tri-color separation and composition optical system 25 between a group of liquid crystal panels 21, 22, and 23 and the projection lens 16, as described hereafter referring to FIG. 3 and FIG. 4, which poses a great difficulty to obtain a large magnified projection image on the screen at a shorter projection distance.
One of the attempts to solve the problem is to provide an optical lens system having a longer back-focal length than a focal length, for instance, called the "Retro-Focus" method.
FIG. 2 is an explanatory view for explaining the Retro-Focus method in the prior art.
Referring to FIG. 2, an optical lens system according to the Retro-Focus method consists of a concave lens group 1 and convex lens group 2 having an overall focal length `f`. After a light 3 coming in parallel with an optical axis 4 is incident to the concave lens group 1 and refracted outward away from the optical axis 4, the light 3 is incident to the convex lens group 2 and refracted inward toward the optical axis 4, so that it can be obtained a back focal length `b.f.` which is longer than the focal length f.
But, in this case of employing the above optical lens system, there exists such a problem that it is difficult to make use of the optical lens system because of inevitable aberration of various kinds. This results in the failure of obtaining a better high definition image, while a large magnified projection image may be obtained at a short projection distance.